The present invention relates generally to insulated coil assemblies and more specifically to an insulated coil assembly, having improved dielectric strength and heat transfer characteristics, for use in electrical apparatus such as dynamoelectric machines.
It is well known in the art to use resin impregnated tapes for insulating electrical apparatus coil assemblies. In such assemblies, an electrical conductor (hereinafter referred to as wire) is normally preformed into a desired coil configuration and the tape is then wrapped around those portions of the coil which are to be electrically insulated from adjacent coils and ground. There may also be an intermediate insulating means such as mica or, NOMEX (trademark of E. I. DuPont Company, Incorporated of Wilmington, Del.) paper which is disposed between the wire and the tape.
When woven glass tape is used in this type of structure, one of two methods is customarily employed. The first of these uses an untreated tape which is wrapped around the preformed wire configuration. This tape is then resin impregnated by a suitable process such as a vacuum-pressure process or by dipping. Because of the nature of untreated tape, this process has the disadvantage of being a very dusty process. A second method which alleviates the dust problem and which serves to increase the strength of the tape is to pretreat the tape with a suitable resin. Because the pretreated tape tends to be tacky and moist, greater problems are associated with handling of the tape, whether by machine or human. The pretreated tape also tends to give off vapors from the resins (and any solvents employed) which may be objectionable to workers performing a hand wrapping process.
In both of the the above methods, after wrapping the tape the coil assembly is usually subjected to an additional impregnating process, such as dipping or vacuum-pressure impregnation. The resulting structure, by these methods, has a tendency to be non-uniform in its coating and often includes voids. These deficiencies are primarily the result of, in the case of the untreated tape, organic materials which tend to preclude proper impregnation. In the pretreated tape case, the pretreat resin in the tape acts to block good penetration of the resin in the final impregnation step. In both cases the non-uniformity of the coating reduces the dielectric strength and the heat transfer capability of the coil assembly.
Use of "heat-cleaned" woven glass tape is known. A heat-cleaned tape is one which has been heated to a temperature sufficient to remove at least some portion of the organic materials (primarily starches) from the tape. Because a heat-cleaned tape tends to develop a tan appearance, it is sometimes referred to as "caramelized tape". One example of the use of a heat-cleaned tape is found in U.S. Pat. No. 4,392,070, "Insulated Coil Assembly and Method of Making Same" by Joseph J. Zdaniewski, which patent was issued on July 5, 1983 and is assigned to the assignee of the present invention. In this patent, an open weave (leno weave) heat-cleaned glass tape is wrapped around a wire coil and this combination is dipped into a viscous compound to provide an insulated structure. The tape strands, in this case, are used to support and reinforce the viscous compound which is normally of an epoxy nature. The result is a thick layer which has reasonable dielectric strength but is not especially heat conductive. This type of construction results in a structure which is very strong and crack resistance but, because of the thickness of the coating, is not particularly desirable for use in smaller devices or smaller confines where space is critical.